A. Field of the Invention
This invention relates to the field of art of control of the length of a welding arc.
B. Prior Art
In the welding art it has been desired to accurately control the length of the welding arc. This is a particularly important problem in automatic machine welding in which welds at high production rates are made by machine operators rather than by welder craftsmen. For example in the volume welding of artillery shells the arc length must be controlled to provide sufficient heat in order to melt the welding electrode wire and auxiliary wire and to heat the outer layer of artillery shell base metal to the melting point of the artillery shell overlay material. The arc energy must be carefully controlled to assure intermetallic bonding without actual mixing the two metals.
Thus, for example, when the arc length has been shorter than optimum, the heat of the arc has been concentrated in a smaller than optimum area thereby providing an undesirably high temperature to the base metal. Accordingly, the arc would dig in, deeply penetrate the shell and undesirably result in iron dilution in the overlay material.
It has previously been believed that arc voltage has determined arc length. However, the arc voltage that has been actually measured was the power source voltage which is the total voltage between the point where the welding electrode wire is coupled to current source and the bottom of the arc or workpiece. That source voltage has been made up of a voltage portion heating the welding wire and a voltage portion maintaining the arc length where these voltage portions have been unknown factors as they have been continually changing. These changes are caused, for example, by the welding electrode wire being coupled to the current source by contact with a long contact tube at a point of entry or point of exit or any place in between. Other voltage variations have been caused by the use of welding electrode wire made of differing metals with various electrical resistance. Further, the surface resistance of welding electrode wire may change as a result of the surface quality of the wire. The contamination of the wire's exterior causes a voltage variation which causes a change in arc voltage and thus arc length.
Accordingly, an object of the present invention is a system for automatically controlling the length of a welding arc.